Ultraviolet resistive antireflective coating

Abstract: A coating for an optical component is formed to be antireflective for a selected wavelength and resistive to the transmission of ultraviolet radiation. The coating has first layer (24A,24B) is formed of a dielectric material having a refractive index n₁ coated onto the optical component. A second layer (26A,26B) is formed of a dielectric layer having a refractive index n₂ that is smaller than the refractive index n₁ of the first layer. The second layer (26A,26B) is formed of a material that transmits the selected wavelength while blocking ultraviolet light, thereby protecting the first layer (24A,24B) and the optical component (12) from damage due to exposure to ultraviolet radiation. The thicknesses of the layers is selected so that the coating is antireflective for the selected wavelength.

Description: BACKGROUND OF THE INVENTION This invention relates generally to optical coatings for controlling the reflection and transmission of particular optical wavelengths at an optical surface. This invention relates particularly to an optical element having an ultraviolet resistive, antireflective coating for a component of an optical system. Still more particularly, this invention relates to an ultraviolet resistive, antireflective coating for an intra-cavity element of a multioscillator ring laser gyroscope. A ring laser gyroscope employs the Sagnac effect to measure rotation. Counterpropagating light beams in a closed path have transit times that differ in direct proportion to the rotation rate about an axis perpendicular to the plane of the path. In a ring laser gyroscope the closed path is defined by mirrors that direct the light beams around the path. The path is generally either square or triangular in shape, although any closed polygonal path could, in principle, be used. The closed path is typically in a cavity formed in a frame or body that is formed of a glass ceramic material. The cavity is evacuated and then filled with a mixture of helium and neon, which is the gain medium for the laser. An electrical discharge excites the gain medium, which produces light amplification and a coherent light source. The mirrors must be precisely aligned to direct the light beams around the closed path in a manner that promotes proper operation of the laser. The mirror surface must be free of impurities to provide a laser beam intensity that will result in a usable signal. Once laser oscillation occurs in the system at resonant frequencies, the difference in the length of the pathways traversed by the counterpropagating laser beams result in a difference or beat frequency which is sensed by a photodetector and amplified by an amplifier. The beat frequency is a result of optically heterodyning the counter propagating beams. It is well-known that a ring laser gyroscope that employs two counterpropagating waves is subject to the phenomenon of lock-in at low rotation rates. At rates below a threshold value, a two-mode ring laser gyroscope will erroneously indicate that the rotation rate is zero. When the frequency difference between the waves is too small, the frequencies appear to be equal, which gives an erroneous indication of a zero rotation rate. One type of ring laser gyroscope that eliminates lock-in is the Zeeman ring laser gyroscope (ZLG). The ZLG has two pairs of (counterpropagating) circularly polarized waves propagating in the resonant cavity simultaneously. One pair of counterpropagating waves consists of right circularly polarized light waves propagating in the clockwise and counterclockwise directions. The other pair consists of left circularly polarized waves which are also propagating in the clockwise and counterclockwise directions within the same resonant cavity. Such a four mode ring laser gyroscope configuration is described in detail in U.S. Patent 3,741,657, issued June 26, 1973 to Enduring and in U.S. Patent 4,213,705, issued July 22, 1980 to Sanders. Operation of a four mode laser gyroscope is briefly described below. Disposed in the path of the propagating waves within the cavity are reciprocally anisotropic and non reciprocally anisotropic dispersive elements. A reciprocally anisotropic dispersive element, such as an optical rotator made of crystalline quartz, provides different propagation time delays (or different optical indices) to right and left circularly polarized waves. This difference in optical index due to the state of polarization results in an optical path length difference between oppositely polarized waves resonating within the same cavity. The non reciprocally anisotropic dispersive element, for example a Faraday cell, presents different refractive indices for light waves traveling in opposite directions. The waves traveling in the counterclockwise and clockwise directions thus have different propagation time delays in the Faraday cell. This difference in propagation time delays produces different path lengths for light waves traveling in opposite directions. Therefore, the combination of the two types of anisotropy can be used to adjust the frequency separation between resonant modes such that all four modes resonate at different frequencies Separation between the resonant mode frequencies is accomplished so that the resonant frequencies of the two waves traveling in one direction are spaced between the resonant frequencies of the two waves traveling in the opposite direction. The two highest frequency modes have the same circular polarization but opposite directions of propagation. Likewise, the two lowest frequency modes have the same circular polarization, opposite from the state of polarization of the other pair and are also counter rotating in the cavity. Each pair of like-polarized modes operates in a separate two mode laser gyro. As the ring laser system is rotated about an axis perpendicular to the plane of the propagating waves, the frequency separation between the two higher frequency modes will either decrease or increase wile the frequency separation between the two lower frequency modes will either increase or decrease. The output beat signal resulting from combining the two lower frequency modes is subtracted from the output beat signal resulting from combining the two higher frequency modes. This produces a substantially linear representation of the rotation rate of the laser system. The direction of rotation is determined by monitoring one of the pair of modes. The intracavity element presents difficulties in using a ZLG for rotation sensing. The cavity is basically evacuated and has a refractive index of about 1.00. The intracavity element has a refractive index that is greater than that of the optical medium in the cavity. Therefore, reflections occur when the waves impinge upon the intracavity element. These reflections cause undesirable mode coupling and resonator losses, which reduce the accuracy of the output from the ZLG. Antireflective coatings have been used on intracavity elements to reduce the amount of light reflected by the intracavity elements. These coatings have had to meet stringent requirements for thickness. The prior art procedure for fabricating such coatings involves depositing the first layer to a thickness greater than required, measuring the thickness of the first coating, and then milling down to the desired thickness. Difficulty in satisfying the requirements for thickness of the coatings causes errors and increases the cost and time for constructing a suitable intracavity element. There are several prior art antireflective coatings. For example, US Patent 4,966,437, issued October 30, 1990 to Rahn and US Patent 4,907,846, issued March 13, 1990 to Tustison et al. describe previous antireflective mirror coatings. A coated optical element for use in N 2 - or HeCd-gas or XeCl excimer lasers comprising a substrate of SiO 2 coated with a first layer of Ta 2 O 5 doped with Al 2 O 3 and a second layer of Al 2 O 3 is disclosed in EP 0 379 738. SUMMARY OF THE INVENTION The present invention provides for an intracavity element having a two-layer coating which coating is easy to fabricate and which has reduced tolerance on the thickness requirements for the inner layer. According to the present invention, there is provided an optical element that comprises a block of crystalline SiO 2 having a first and a second surface opposite to each other, the first surface being coated with: a first layer formed of a first dielectric material that comprises Ta 2 O 5 doped with Al 2 O 3 or TiO 2 and having a refractive index n 1 and a second layer formed of a second dielectric material that comprises Al 2 O 3 and having a refractive index n 2 that is smaller than the refractive index n 1 of the first dielectric material to transmit a selected wavelength of light while blocking ultra-violet light, thereby protecting the first layer and the first surface of the block of crystalline SiO 2 from damage due to exposure to ultra-violet radiation; and characterised by a third layer formed of the first dielectric material coated onto the second surface of the block of crystalline SiO 2 ; and a fourth layer formed of the second dielectric material coated onto the third layer, the fourth layer protecting the third layer and the second surface of the block of crystalline SiO 2 from damage due to exposure to ultra-violet radiation, the first and second layers and the third and fourth layers having thicknesses and refractive indices n 1 and n 2 that co-operate to make the optical element anti-reflective for light of the selected wavelength incident upon the surfaces of the second and the fourth layers opposite the first and the third layers, respectively. Preferably the Ta 2 O 5 of the first and third layers is doped with Al 2 O 3 or TiO 2 to a degree appropriate to produce a zero birefringence shift at the selected wavelength. Preferably, where the Ta 2 O 5 is doped with Al 2 O 3 , it is doped with Al 2 O 3 to a degree appropriate to produce a refractive index n 1 of about 1.98. An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The Figure illustrates an intracavity element for a Zeeman laser gyroscope having an antireflective coating applied thereto. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the Figure, an intracavity element 10 for a Zeeman ring laser gyroscope (not shown) comprises a block 12 of a material that produces the desired circular polarization-dependent time delays for light waves incident thereon. The light waves are indicated by the arrows labelled CW BEAM and CCW BEAM. As explained above, the intracavity element 10 should have an antireflective coating 14 for the optical wavelength that is amplified by the gain medium (not shown) in the ZLG. It is well-known that antireflective coatings may be formed by placing one or more layers of a dielectric of appropriate refractive index and thickness on the surfaces 16 and 18 of the block 12 where the laser light is incident. A two layer antireflective coating 14 includes a first layer 24A coated onto the surface 16 of the block 12. The layer 24A preferably is formed from a high refractive index material. A second layer 26A is coated onto the first layer 24A. The second layer 26A is formed from a material having a refractive index smaller than that of the refractive index of the first layer 24A. The two layer coating 14 is commonly known as a V-coat. It has a very narrow acceptance angle for the wavelengths that exist in the ZLG. The preferred gain medium in the ZLG is a mixture of helium and neon that lases at 632,8 nm (6328Å). The block 12 preferably is formed of crystalline silicon dioxide, SiO 2 , which has a refractive index of about 1.46. The high refractive index layer 24A is tantala, Ta 2 O 5 , that has been doped with alumina. Tantala has a refractive index of about 2.05. The tantala is preferably doped with the alumina to achieve a refractive index of about n 2 = 1.98. The low refractive index layer 26A is formed on the high refractive index layer 24A. The outer layer 26A is formed of Al 2 O 3 having a refractive index n 2 = 1.65, which blocks ultraviolet radiation and prevents UV damage to the layer 24A and the block 12. Each layer should be a quarter wave plate. Therefore, the thickness, T, of each layer is equal to λ / 4n where n is the refractive index of the layer material and λ is the wavelength of the light in vacuum. It is assumed that the light is normally incident on the intracavity element 10. Similarly high and low refractive index coatings 24B and 26B are formed on the surface 18 of the block. One of the counterpropagating waves, for example the clockwise wave, is incident upon the layer 26A and is transmitted through the layers 26A and 24A, the block 12, and the layers 24B and layer 26B without reflection. The other wave is incident upon the layer 26B and is also transmitted without reflection. The two layer coating 14 formed according to the present invention has the advantage of allowing about a 16% tolerance in the thickness of the two inner coatings 24A and 24B. This advantage arises from the relationship between the refractive indices of Ta 2 O 5 :Al 2 O 3 and Al 2 O 3 . Because of the relatively small difference between the refractive indices of the two layers, a small variation in the thickness of the inner layer 24A formed of Ta 2 O 5 :Al 2 O 3 does not produce a significant effect on the antireflective nature of the coating 14. The outer coatings 26A and 26B may be milled, if necessary, to adjust the antireflective property of the composite coatings to a specified frequency. Oxides such as SiO 2 , TiO 2 , HfO 2 , ZrO 2 , Ta 2 O 5 , for example, exhibit a shift in birefringence under prolonged exposure to ultraviolet radiation. The direction of the shift in birefringence depends upon the material. The birefringence shifts in Ta 2 O 5 :Al 2 O 3 and Al 2 O 3 , in particular, are opposite. This invention uses Ta 2 O 5 doped with Al 2 O 3 to a degree appropriate to produce a zero birefringence shift for the 632,8 nm (6328Å) wavelength in the ZLG. Other materials having opposite birefringence shifts may also be combined to produce a layer having essentially zero shift in birefringence upon UV exposure. One such combination of materials is a mixture of TiO 2 and Ta 2 O 5 to produce the high index layer. The material of the low index layer is Al 2 O 3 . The outer layer formed of Al 2 O 3 blocks ultraviolet radiation and prevents UV damage to the inner layer. The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning of the claims are embraced within the scope of the invention.

Claims: An optical element (10) that comprises a block of crystalline SiO 2 (12) having a first (16) and a second (18) surface opposite to each other, the first surface (16) being coated with: a first layer (24A) formed of a first dielectric material that comprises Ta 2 O 5 doped with Al 2 O 3 or TiO 2 and having a refractive index n 1 and a second layer (26A) formed of a second dielectric material that comprises Al 2 O 3 and having a refractive index n 2 that is smaller than the refractive index n 1 of the first dielectric material to transmit a selected wavelength of light while blocking ultra-violet light, thereby protecting the first layer (24A) and the first surface (16) of the block of crystalline SiO 2 (12) from damage due to exposure to ultra-violet radiation; and characterised by a third layer (24B) formed of the first dielectric material coated onto the second surface (18) of the block of crystalline SiO 2 (12); and a fourth layer (26B) formed of the second dielectric material coated onto the third layer (24B), the fourth layer (26B) protecting the third layer (24B) and the second surface (18) of the block of crystalline SiO 2 (12) from damage due to exposure to ultra-violet radiation, the first and second layers (24A, 26A) and the third and fourth layers (24B, 26B) having thicknesses and refractive indices n 1 and n 2 that co-operate to make the optical element (12) anti-reflective for light of the selected wavelength incident upon the surfaces of the second and the fourth layers opposite the first and the third layers, respectively. An optical element as claimed in Claim 1, further characterised in that the Ta 2 O 5 of the first and the third layers (24A, 24B) is doped with Al 2 O 3 or TiO 2 to a degree appropriate to produce a zero birefringence shift at the selected wavelength. An optical element as claimed in Claim 1 or Claim 2, further characterised in that, where the Ta 2 O 5 is doped with Al 2 O 3 , it is doped with Al 2 O 3 to a degree appropriate to produce a refractive index n 1 of about 1.98. The use of the optical element claimed in any of Claims 1 - 3 as an intracavity element in a ring laser gyroscope. The use of the optical element claimed in any of Claims 1-3 as an intracavity element in a Zeeman ring laser gyroscope operating at said selected wavelength, said selected wavelength being of 632.8 nm.

Inventors: LOUDERBACK ANTHONY W, LU SAMUEL, STEWART ALAN F, SUN MING-JAN, LOUDERBACK, ANTHONY W., LU, SAMUEL, STEWART, ALAN F., SUN, MING-JAN
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1994
Main classification: G02B1
IPC codes: H01S3, G01P9, G01B9, G02B1, G02B5, G01C19